This invention relates to a process for preparing a polyorganosilane or a silicon-containing polymer, more specifically to a process for preparing a polyorganosilane, in which a polyorganosilane can be prepared with good efficiency, its molecular weight can be controlled easily and optional various organic groups can be introduced into a polyorganosilane easily.
It has been known that a polyorganosilane is useful as an organic conductive material, a non-linear optical material, a photolysis type reaction initiator, a silicon carbide precursor and a photoresist material (e.g., Miller, R. D.; Michl, J., Chem. Rev., 89, 1359 (1989)). As a process for preparing a polyorganosilane, the Wurtz method using an organohalosilane or an organohalodisilane as a starting compound has been used widely. However, the Wurtz method has drawbacks that danger is incurred because metallic sodium or metallic potassium having self-ignition property in the air is used; reaction conditions are severe; it is difficult to control a molecular weight so that a polymer having a bimodal molecular weight distribution is produced; and it is difficult to introduce substituents other than an alkyl group and an aryl group.
Some attempts have been made to solve the above drawbacks. For example, there have been obtained by Nagai et al. cyclic dodecamethylcyclohexasilane and .alpha.,.omega.-dialkoxydisilane having a low molecular weight by disproportionation reaction of 1,2-dialkoxytetramethyldisilanes in the presence of sodium alkoxide without using metallic sodium nor metallic potassium (Japanese Provisional Patent Publications No. 24874/1979 and No. 146790/1982). There have been also reported syntheses of a high molecular weight polysilane by using disproportionation reaction of a disilane having an alkoxy group in the presence of sodium alkoxide as described above (Japanese Provisional Patent Publications No. 59183/1993 (which corresponds to U.S. Pat. No. 5,023,307) and No. 59184/1993 (which corresponds to U.S. Pat. No. 5,025,075), Japanese Chemical Society, 61st Spring Meeting: IG328, IG329 (1991), H. Watanabe et al., J. Mater. Chem., 1,483 (1991) and Japanese Provisional Patent Publication No. 311727/1992).
In these reactions, an alkali metal hydrocarbyloxide synthesized from an alcohol or a phenol and an alkali metal such as sodium or synthesized from an alcohol or a phenol and an organic alkali metal reagent such as butyl lithium is generally used as a reaction catalyst. The reactions are achieved by adding a previously isolated solid alkali metal hydrocarbyloxide as a catalyst or by a catalyst previously prepared in a flask in which disproportionation reaction is carried out. However, synthesis of such a common alkali metal hydrocarbyloxide is complicated and should be carried out with care.
That is, in general, when an alkali metal hydrocarbyloxide is synthesized, an excessive amount of the above alcohol or phenol is used in many cases and, if necessary, an organic solvent is further used. In order to obtain a polysilane by subjecting a disilane to disproportionation reaction, it is necessary to completely remove the alcohol or phenol used in an excessive amount and the organic solvent used, if necessary. Further, a solid alkali metal hydrocarbyloxide is obtained by removing the alcohol or phenol and low boiling point compounds such as the organic solvent so that the solid alkali metal hydrocarbyloxide is required to be dissolved in the disilane for carrying out disproportionation reaction. However, solubility of the alkali metal hydrocarbyloxide which becomes solid once is not always good so that a time for resolving it is required in some cases. Further, the alkali metal hydrocarbyloxide after removing a solvent therefrom is easily changed in quality during storage for a long period of time. When it is changed in quality, solubility is further lowered so that a catalyst activity to disproportionation reaction is lowered greatly. Thus, handling in the case of using the alkali metal hydrocarbyloxide as a catalyst of disproportionation reaction is complicated.
Further, various methods of obtaining an organic silicon polymer having a polysilane chain have been attempted. For example, there has been obtained by Kashizaki et al. a polymer having a potysilane chain which is soluble in an organic solvent, by subjecting diorganodichlorosilane and .alpha.,.alpha.'-dichloroxylene to the Wurtz reaction simultaneously (Japanese Provisional Patent Publication No. 139216/1992). Also, there has been obtained by Asuke et al. a silicon-containing polymer in which a benzene ring is introduced into a main chain by reacting diorganodichlorosilane and dilithiobenzene at a ratio of 1:1 (Japanese Provisional Patent Publication No. 342726/1992).
On the other hand, there has been obtained by Shono et al. a silicon polymer having an organic group and a weight average molecular weight of about 5,000 to 18,000 in which an atom or atoms other than silicon is/are introduced into a main chain, by carrying out an electrode reaction using a bis(halosilyl) compound as a starting compound (Japanese Provisional Patent Publication No. 348128/1992). There has been obtained by Ishikawa et al. a polymer having conductivity in which a thienyl group is introduced into a main chain, by polymerizing a bis(5-halomagnesiumthienyl)silane derivative by a nickel catalyst (Japanese Provisional Patent Publication No. 218533/1992). Further, there has been obtained by Ishikawa et al. a polymer in which ethynylene is introduced into a main chain, by subjecting a cyclic silane derivative containing ethynylene to ring-opening polymerization ("Organometallics" 1992, 11, 1604 to 1618). There has been obtained by Yamashita et al. a silicon-containing polymer in which a quinone is inserted into a Si--Si bond, by reacting a disilanylene polymer and a quinone ("Macromolecules" 1993, 26, 2143 to 2144).
The disproportionation reactions using alkoxydisilane compounds as described above are excellent in the point that the reaction can be carried out under mild conditions without using metallic sodium nor metallic potassium. Heretofore, several straight silicon-containing polymers have been obtained. However, when application to a non-linear optical material and a photoresist, particularly a conductive material is taken into consideration by making a molecular skeleton have a branched or network structure, an energy band gap between HOMO and LUMO of a polymer is narrowed to heighten utilizability as a conductive material. Also, by heightening a three dimensional element of the structure, heat resistance of the polymer itself can be improved. When such application is taken into consideration, it has been demanded to synthesize various silicon-containing polymers having a branched or network structure and mainly comprising a polysilane bond easily.
It might be possible to obtain various silicon-containing polymers as described above by carrying out a reaction by introducing desired organic groups into a starting silane derivative as carried out by Ishikawa et al. described above. However, When such a silane derivative is obtained, there are problems that it is difficult to carry out a reaction in which organic groups are selectively introduced into the silane derivative and it is difficult to purify the resulting silane derivative. Further, in the reactions of Asuke et al. and Kashizaki et al. large amounts of metallic lithium and sodium should be used so that the reactions cannot be used for industrial mass production.
On the other hand, a chlorosilane compound having various organic groups is easily available, but there has not been known a method of obtaining a silicon-containing polymer having a branched or network structure and mainly comprising a Si--Si bond as described above from the chlorosilane compound safely and easily.
It has been known that in a silicon-containing polymer mainly comprising a Si--Si bond, its average molecular weight determines physical properties and characteristics of the silicon-containing polymer, and its control is extremely important. Particularly when good film-forming property is to be imparted, it is necessary to control its average molecular weight to 1,000 or more and also in such a range that said polymer has a solubility enough to prepare a 0.1 to 50% by weight, preferably 5 to 40% by weight solution. Therefore, there has been demanded a method of controlling the average molecular weight of such a silicon-containing polymer easily.
Also, organic groups bonded to a silicon atom of the silicon-containing polymer determine physical properties and characteristics of the polymer so that selection of the organic groups is important. Therefore, there has been demanded a method of introducing selected various substituents to such a silicon-containing polymer easily.
There has been obtained by Watanabe et al. a white powdery polysilane which has a methoxy group and is insoluble in toluene, by disproportionation reaction of 1,2-dimethyl-1,1,2,2-tetramethoxydisilane by using n-butyl lithium as a catalyst (see Japanese Chemical Society, 61st Spring Meeting: IG328, IG329 (1991)). However, there has not been described preparation of a catalyst further suitable for disproportionation based on such an organic alkali metal compound and also there have not been disclosed control of the average molecular weight of a polysilane and introduction of various organic groups demanded as described above, using such a catalyst.